How grannies evolved into a busy mother's best friend

But in far distant times, it seems grandmothers performed a far more important function - helping the human race survive and thrive.

And they may have been responsible for the evolution of the menopause, experts have found.

Their research has shown that in less advanced societies, having a grandmother at hand to lavish a newborn with love and attention greatly increased a baby's chances of reaching the age of two.

Using thousands of birth and death records and a series of complex mathematical equations, the British scientists worked out that the optimum age for a woman to go through the menopause is 50.

According to their theory, menopause evolved to create a generation of women who stopped having babies themselves but were still capable of caring for their children's children, benefiting the survival of the community as a whole.

By nurturing her grandchildren, a grandmother safeguards the survival of her genes without having to go through the trauma of continuing childbirth herself.

The theory comes from a study set up to work out why women lose the ability to have children when they are still relatively young, while certain other mammals are able to reproduce into old age.

The researchers, from Newcastle University and University College London, looked at which of two possible explanations was most likely.

The first was that menopause is a way of ensuring women become mothers while they are still young, cutting the risk of them dying while their children were unable to fend for themselves.

The researchers said that too few mothers die when their children are still helpless for this to be the reason for menopause.

However, the second theory - that grandmothers played a key role in the evolution of the menopause - held water. The study showed that if a woman goes through the menopause around 50, her children are much more likely to have the benefit of a grandmother's care.

As a result, the population as a whole is more likely to grow.

Writing in the journal Proceedings of the Royal Society B, the researchers said: "Our results highlight the importance of grandmaternal contribution.

"Our results point to the distinctive and perhaps unique role of menopause in human evolution and provide important support for the hypothesised evolutionary significance of grandmothers."

The researchers came to their conclusions after manipulating figures gleaned from birth and death records of more than 5,500 Gambian villagers.

The data was collected between 1950 and 1975, before a medical centre was established in the area.

The figures, which were unaffected by the availability of contraception or the sort of healthcare available in the developed world, were used as a guide to the birth and survival rates experienced by our huntergatherer ancestors.

If grandmothers today are playing a smaller role in bringing up their children, it could be argued it is no longer necessary for women to stop having babies when they hit the menopause.

Researcher Dr Daryl Shanley said IVF techniques meant modern-day women had the option of 'overcoming the biological limits of fertility'.

In the UK, the average age for the menopause - defined as the time when a woman's periods have stopped for 12 months - is 52. However, one per cent of British women go through the menopause before they reach 40.